Due to the ever increasing amount of data being exchanged globally, there is a need for systems and methods enabling faster transmission of data, wirelessly as well as through various types of wires.
For example, the capacity of fiber-optical communication systems has so far increased exponentially, mainly due to hardware improvements—better fibers, lasers, detectors, amplifiers, etc, are being developed.
As a complement to improvements in hardware, system designers are also looking for other options for further improving the data transmission capacity, since, although further improvements in hardware is probably possible, the cost is expected to be rather high in relation to the resulting improvements in data transmission capacity. A similar development has to a large extent already taken place in wireless communications.
One such other option for further improving the data transmission capacity is to use advanced modulation formats. In so-called I/Q modulation, both the amplitude and phase of the electromagnetic wave are used, which increases the transmission capacity, but unfortunately also the receiver complexity. More specifically, the receiver needs to be supported by a synchronization module, which by ND conversion and signal processing recovers a phase reference from the data signal. I/Q modulation is included in many communication standards, but synchronization difficulties have so far prevented it from reaching the market for applications with very high data rates, such as fiber-optical communication systems.
Another method to increase the data transmission capacity of a data transmission system exploits the fact that electromagnetic waves can be decomposed into two independent polarizations. By sending data in both polarizations, and detecting them independently of each other, the capacity can be doubled. This technique is already in use in some wireless systems. It has also been demonstrated experimentally over optical fibers and seems to be ready for commercial deployment soon. This concept is illustrated in FIG. 2a, where x and y represent the two polarization planes. The figure also shows how data is transmitted with different wavelengths λ in order to utilize the full available electromagnetic spectrum.
Instead of sending data in both polarizations, one may transmit data in one polarization and a pilot tone (an unmodulated carrier, i.e., a pure sinusoid) in the other polarization, such as is, for example, described in U.S. Pat. No. 7,421,210. This method is sometimes referred to as “self homodyne”. A similar known data transmission scheme, which also includes wavelength multiplexing is illustrated in FIG. 2b. The purpose is that the pilot tone can serve as a phase reference for the data signal if both have the same wavelength and are in phase, which makes it possible to use I/Q modulation without any synchronization in the receiver, which means that the configuration of the receiver becomes simpler and thus potentially less costly. Although the system in FIG. 2b contains half the number of data signals as compared to the system of FIG. 2a, more advanced modulation formats (higher data rates) in each data signal allows the system of FIG. 2b to reach the same total capacity as the system of FIG. 2a, or higher. It would, however, be desirable to achieve an even higher data transmission capacity without significantly increasing the complexity of the data transmission system.